The D-proline of formula II (parent compound) is a known compound and is disclosed in EP 915 088. Formula II is provided as 
Compounds of formula II have a limited bioavailability. It was therefore useful to find derivatives of the compound of formula II to render these compounds suitable for oral application.
A molecule with optimal structural configuration and physicochemical properties for eliciting the desired therapeutic response at its target site does not necessarily possess the best molecular form and properties for delivery to its point of ultimate action. Usually, only a minor fraction of doses administered reach the target area and since most agents interact with non-target sites as well, an inefficient delivery may result in undesirable side effects. This fact of differences in transport and in situ effect characteristics for many drug moleculs is the fundamental reason why bioreversible chemical derivatization of drugs, i.e, prodrug formation, is a means by which a substantial improvement in the overall efficacy of drugs can be achieved.
Therefore, the prodrug approach involves    1. enhancement of bioavailability and passage through various biological barriers,    2. increased duration of pharmacological effects,    3. increased site-specificity,    4. decreased toxicity and adverse reactions,    5. improvement of organoleptic properties, and    6. improvement of stability and solubility.
A prodrug is a pharmacologically inactive derivative of a parent drug molecule that requires spontaneous or enzymatic transformation within the body in order to release the active drug, and that has improved delivery properties over the parent drug molecule. Prodrugs are designed to overcome pharmaceutically and/or pharmacokinetically based problems associated with the parent drug molecule that would otherwise limit the clinical usefulness of the drug.
In recent years several types of bioreversible derivatives have been exploited for designing prodrugs. Using esters as a prodrug type for drugs containing carboxyl or hydroxyl function is most popular. Further well-known are prodrug derivatives of peptides, 4-imidazolidinones and the like, described in Drugs of the Future, 1991, 16(5), 443-458 or N-oxides, described, for example, in U.S. Pat. No. 5,691,336.
It is desirable to provide novel compounds of formulas I and IA to overcome pharmaceutically and/or pharmacokinetically based problems associated with the parent drug molecule of formula II that would otherwise limit the clinical usefulness of the drug.